Collocation methods for differential-algebraic equations of index 3

Numer. Math. 65, 407-421 (1993)

Numerische MathematJk

9 Springer-Verlag1993

Collocation methods for differential-algebraic equations of index 3 Laurent Jay Universit~ de Gen~ve, D~partement de math~matiques, Rue du Li~vre 2-4, Case postale 240, CH-1211 Gen+ve24, Switzerland Received May 14, 1992 / Revised version received February 8, 1993

Summary. This article gives sharp convergence results for stiffly accurate collocation methods as applied to differential-algebraic equations (DAE's) of index 3 in Hessenberg form, proving partially a conjecture of Hairer, Lubich, and Roche.

Mathematics Subject Classification (1991): 65L06

1. Introduction Index 3 problems often appear when modelling mechanical systems with constraints (for further details see [1, Sect. 6.2], [3, pp. 6-7] or [5, pp. 483-486 and pp. 539-540]). A usual way for solving such problems is by index reduction (see [-1, Subsects. 2.5.3 and 5.4.1]). However, for multibody systems containing rigid springs with a Hooke's constant 1/e2 (e very small), the numerical solution behaves as that for the limit problem (e ~ 0 ) which is of index 3 (see [3, pp. 10-12] or [6] ). In this situation, an index reduction is not possible and one is compelled to study the convergence behaviour for the index 3 case. This remark also holds for very stiff mechanical systems in which a large potential forces the motion to be close to a manifold. Convergence of BDF-methods is stated in [-1, Subsect. 3.2.4]. Preliminary theoretical convergence results for general implicit Runge-Kutta (IRK) methods have been obtained in [-3, Sect. 6], but numerical experiments have shown that they are not optimal and sharper orders of convergence have been hypothesized [-3, pp. 18-19 and p. 86]. For solvable linear constant coefficients systems of arbitrary index, necessary and sufficient conditions to ensure that the local and global errors of an I R K method attain a given order of accuracy have already been derived in [2]. The main result of this paper (Theorem 2.2 below) will be a partial proof of the conjecture of [3, p. 86], giving sharp convergence bounds for stiffly accurate collocation methods, such as the Radau IIA processes. The necessary tools for this proof (Sect. 5), which relate on the ideas of [-3, Sect. 6], and of [5,

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L. Jay

Sect. VI.7] (devoted to index 2 systems), are collected in Sect. 3 (existence, uniqueness, and study of perturbations in the initial values) and in Sect. 4 (local error).

2. Collocation methods for index 3 DAE's

Let (2.1)

y' =f(y, z),

y(xo)=Yo~",

z'=k(y,z,u),

Z(Xo)=ZoelR",

0-----g(y),

U(Xo)=Uoe]R p

be a system of differential-algebraic equations given in an autonomous and semi-explicit formulation (or Hessenberg form). The initial values (Y0, Zo, u0) are assumed to be consistent, i.e., to satisfy (2.2a)

0=g(y)

(2.2b)

0 = (gyf) (y, z)

(2.2 c)

0 = (gyy( f , f ) + gyfr f + gy f~ k) (y, z, u).

Let us suppose that f, g, and k are sufficiently differentiable functions and that (2.3)

gyf~ k,

is invertible

in a neighbourhood of the exact solution (index 3). If (2.2c) and (2.3) are satisfied for some (y*, z*, u*), then, in a vicinity of these values, (2.2c) defines an implicit function u = G(y, z). One step of a collocation method applied to (2.1) is defined as follows: Definition 2.1. Let c 1. . . . , cs be s distinct real numbers and let (Y(x), Z(x), U(x)) denote the collocation polynomials of degree s which satisfy

(2.4 a)

Y(xo) = Yo,

Z (Xo) = Zo,

U (Xo) = Uo,

(2.4b) Y'(xo + ci h) = f ( Y ( x o + ci h), Z(xo + ci h))

Z'(x~176176

U(x~

I I i = 1 , ...,s.

0 = g (Y(Xo + ci h)) Then the numerical solution is given by (2.4c)

Yl = Y(xo+h),

zl =Z(xo+h),

Ux = U(xo+h).

Remark. The condition U(xo)=Uo in (2.4a) can be omitted if we require U(x) to be a polynomial of degree s - 1 only, and U(xo) to be close to Uo, i.e., U(xo) -Uo=O(1). This modification does not alter the polynomials Y(x) and Z(x), and will apply throughout this paper. Consequently the three collocation polynomials become independent of Uo.

Collocation methods for index 3 DAE's

409

In this article we turn our interest to collocation methods with s > 2 and coefficients satisfying the hypotheses HI: ci+0

for i-- 1, . . . , s ;

H 2 : cs = 1, i.e., the method is stiffly accurate. It can be noticed that from H 2 we get g ( y 0 = 0 in (2.4). The following theorem gives the optimal orders of convergence for collocation methods satisfying these hypotheses, proving the conjecture stated in [3, p. 86] for such methods: Theorem 2.2. Let us consider the differential-algebraic system (2.1) of index 3 with consistent initial values and the collocation method (2.4) satisfying s > 2, H 1, and H 2. Then, for x, - Xo = n h < Const, the global error satisfies

(2.5)

Yn -- Y (Xn) = O (h rain (p" 2 5 - 2)),

Pz (Xn) (Zn -- Z (Xn)) = O (h rain (p" 2 s - 2)),

z.-- z(x.)= O(hS),

u . - u ( x . ) = O(h ~- 1)

where P~(x) = (I - k.(grf~ ku)- 1grf~)(y(x), z(x), u(x)) and p is the order o f the underlying quadrature formula. I f in addition the function k o f (2.1) is linear in u then we get

(2.5')

y, - y (x,) = 0 (hP),

I'~(x,) ( z . - z (x.)) = 0 (hV).

The proof will be presented in Sect. 5, and it makes use of preliminary results contained in Sects. 3 and 4. The result (2.5) shows that if in a last step the numerical solution is projected onto the manifold (gyf)(y, z ) = 0 , the accuracy of the z-component can be improved. This projection can be d o n e as follows: ~. and /~. are the solution of (2.6)

~, = z. + ku (y,, z., u,) #, 0 = (gyf) (y,, ~.)

where (y,, z., u.) is the numerical solution of (2.4) at x.. In this case we get (2.7)

A

, ,

f O (h p)

z, - z tx,) = ~10 (h mi"tp' 2s -

2))

if k is linear in u, else.

Furthermore, if we define ~. as the solution together with y. and ~. of (2.2c), we have (2.8)

a, - u (x,) = ~ 0 (h e) ~O (h min(p' 2s- 2))

if k is linear in u, else.

The application of the above results to the R a d a u IIA processes leads to:

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L. Jay

Corollary 2.3. For the s-stage (s>2) Radau IIA method applied to the index

3 system (2.1), the global error satisfies

(2.9)

Yn - y(x,) = O (h 2s- 2),

z,,-- z (x,,) = 0 (hS),

P~(x.) (z,, -- z (x.)) = 0 (h 2~- 2), ~,, - z (x,,) = 0 (h 2~- 2), a . - u ( x . ) = 0 (h 2 s - 2)

u . - u ( x . ) = 0 (h ~- 1),

and if k is linear in u we have

(2.9')

y . - y(x.) = O(h 2~- 1), zn-- z(x,) = O(h 2s- 1),

Pz(X.) ( z . - z(x.)) = O(h zs- '), ll.--u(x.)=O(h2S-1).

Proof The proof is obtained by putting p = 2 s - 1 (2.8). []

in (2.5)-(2.5'), (2.7), and

3. Existence, uniqueness, and influence of perturbations

In this section, (Yo, z0, Uo) in Definition 2.1 are replaced by approximate h-dependent starting values (~/, (, v). We will first investigate the existence and uniqueness of the collocation polynomials. T h e o r e m 3.1. Let us suppose that s>=2, H 1 is satisfied, (2.3) holds in a neighbour-

hood of (q, ~, v), and that

(3.1a)

g(r/) = O(h~),

(3.1b)

(grf)(q,O=O(h~),

(3.1 c)

z>3, re>2,

(g,, (f, f ) + g, f, f + g, f~ k) (q, (, v) = O (h).

Then for h < h o the collocation polynomials (Y (x), Z(x), U (x)) of (2.4) with Y (xo) =q and Z ( x o ) = ( exist and are locally unique. Proof A straightforward extension of [4, Theorems II.7.6 and 11.7.7] to index 3 problems shows the equivalence of (2.4) with an s-stage IRK method. More precisely, the values Y (xo + c~h), Z (xo + ci h), and U (xo + c~ h) can be interpreted as the internal stages of a R K method whose coefficients, depending on the c~, are defined by the simplifying assumptions B(s) and C(s) (see [3, pp. 15-16] or [5, Sect. IV.5]). H1 ensures that the corresponding R K matrix is invertible. By defining Co.'=0, the collocation polynomials are uniquely determined by

(3.2a)

Y(x o + th)=

L li(t) Y(xo + ci h)= lo(t) rl + L li(t) Y(xo + ci h) i=O

(3.2b)

Z(xo + th) =

i=1

L li(t) Z(xo + ci h)= lo(t) ~ + L li(t) Z(xo + ci h) i=0

(3.2 c)

i=l

U (Xo + t h) -- L Li (t) U (Xo + ci h) i=1

Collocation methods for index 3 DAE's

411

where the l~(t) and L~(t) are the Lagrange polynomials of degree s and s - 1 respectively, given by I t-c~ \ li(t)=::lo~),

(3.3)

Li(t)= [ l (t--~-~-I 9 j = 1 \ci -- cfl

jr

j~i

The result is now an immediate consequence of the existence and uniqueness of the R K solution stated in [3, Theorem 6.1]. [] We will study next the influence of perturbations in the initial values on the collocation solution. Theorem 3.2. In addition to the assumptions of Theorem 3.1, let us suppose that

H 2 holds and that (3.4)

O=n+O(h3),

~'= ~ + O(h2).

Let us consider ( Y (x), Z(x), 0 (x)) the collocation polynomials satisfying ~'(Xo)= 0 and 2(Xo) = ~. Then we have (3.5a)

AY(xo + h ) = P r A n + h f z P z A ( +O(h IIAnll + h 2 IIP~A~TI + h m+2 IIQz A(II + IIQz A(II 2)

(3.5b) P~(x0 + h) AZ (x o + h) = Pz A ( + O (llQr Anll + h IIPy Anl[ + h IIP~A(II + h "+1

1 IIQ~Affll+~llQz (3.5c)

A~II2)

O"

Q~(xo+h) AZ(xo+h)= - ~ .SQ, An + O(llAnll +h IIA(tl)

where a is a constant depending on the coefficients of the method and is given in the proof, m = m i n ( z - 3 , x - 2 , s - 2 , m a x ( p - s - I , 0)), n = m i n ( z - 3 , tc-2, s - 2 , p - s ) . The projectors Qy, Pr, Qz, and P~ are defined under the hypothesis (2.3) by

(3.6)

S.'=ku (gr fz ku)-1 gy,

O,'=L S,

g , = I - Q,,

Qz'=Sfz,

P~,=I- Qz.

In (3.5) they are evaluated at (n, ~, G(n, 0) with G described in Sect. 2 and the arguments of Pz(xo+h) and Q~(xo+h) are (Y(xo+h), Z(xo+h), G(Y(xo+h), Z (x o + h))).

412

L. Jay

Remarks. 1)The notation A indicates a difference between a "non-hat value" with the corresponding "hat value", e.g., in (3.5a) AY(xo+h)=Y(xo+h) - ~'(xo+h) and A r / = r / - ~ . 2) The missing arguments for fz, S, Py, etc., are (q, (, G(q, 0). 3) TheAconditions (3.4) ensure the local existence and uniqueness of (~'(x), Z(x), U(x)). 4) If g ( f f ) = 0 - g ( q ) then Q~(~-~/)=O(II(t)-~/N2). Consequently this term may be neglected and the hypothesis t~= t/+ O(h 3) can be relaxed to ff = q + O(h2). 5) If the function k of (2.1) is linear in u, then in (3.5a, b) m - - m a x ( p - s - l , 0), n = p - s , and the terms ttQ~A(fl 2 are multiplied by one additional factor h. 6) It can be noticed that m and n satisfy 0 < m_< n < m + i. 7) The important results consist in the splitting of A ( according to the projections P~ and Qz and in the h-exponents in front of tl(2~A(II in (3.5a, b). 8) We point out that the constants entering in the O(-) terms of (3.5) depend on those implied by the O(-) terms of (3.1a, b) and (3.4). Nevertheless this will not affect the proof of Theorem 2.2 (see Sect. 5) where Theorem 3.2 will be used. Proof. A large number of the ideas contained in this proof are expressed in the demonstrations of [5, Theorem VI.7.9 and Lemma VI.7.10]. The proof is divided into four parts. Our first aim in a) is to show (3.15) with the help of the nonlinear variation-of-constants formula [4, Formula (I.14.20)]. In part b) we analyse in details the two terms entering in (3.15), leading to (3.22)-(3.23). Hence (3.15) can be rewritten as the sum (3.24) of several terms expressed in (3.25). Those are then estimated separately in the last part d) with the help of some technical results derived in c). a) The defect of the collocation polynomials (Y(x), Z(x), U(x)) inserted into the differential-algebraic problem (2.1) is defined as follows

(3.7a)

Y' (x) = f ( Y (x), Z (x)) + 6 (x)

(3.7b)

Z' (x) = k ( r (x), Z (x), U (x)) + S~(x)

(3.7c)

0 = g ( r ( x ) ) + O(x).

According to Definition 2.1 6(x), p(x), and O(x) vanish at the points Xo+Cih. By differentiating (3.7c) twice with respect to x and by taking (3.7a, b) into account, the collocation polynomials and the defect are seen to satisfy for all x

(3.7d) (3.7e)

O=gr(Y)(f(Y, Z)+6)+O' O=grr(Y)(f(~ Z)+6,f(Y, Z)+6)+gr(Y)fr(Y, Z ) ( f (Y, Z)+6) + gr( Y) f~( Y,,Z) (k ( Y, Z, U) + #) + gy( Y) 6' + O".

Furthermore, these equations can be used in a vicinity of the solution of (2.1) for arbitrary (Y,Z, U) and sufficiently small "perturbations". In view of (2.3) U can be extracted from (3.7 e), giving (3.8)

U = G(yZ, 6, 6', ~, 0")

C o l l o c a t i o n m e t h o d s for index 3 D A E ' s

413

and it extends the definition of G(y, z) in Sect. 2 which simply corresponds to G(y, z, 0, 0, 0, 0). Thus, (3.7b) can be rewritten (3.7b')

Z'(x)= k(Y(x), Z(x), G(Y(x), Z(x), 6(x), 6'(x), #(x), O"(x)))+p(x)

forming together with (3.7a) a differential system for Y(x) and Z(x). As it concerns ($'(x), Z(x), O(x)), by straightforwardly following the above analysis with 8(x),/~(x), and O(x) defined in the same way as in (3.7), we obtain (3.9a) (3.9b)

~"(x)=f(~(x),2(x))+8(x) Z'(x) = k(~(x), Z(x), G(~'(x), 2(x), 8(x), 8'(x), f~(x), O"(x)))+ f~(x).

With the aim of expressing AY(x) and AZ(x), the nonlinear variation-of-constants formula of Gr6bner-Alekseev [-4, Corollary 1.14.6] can be applied. The difference between Y', Z' and Y,Z formally inserted into (3.9) needs to be computed. We get (3.10)

d(x, gZ)=( ,'] { f(Y'Z)+~ \zT-!,k(Y, z, G(Y,z, $(x), 3'(x),

0"(x)))+ p(x)

= q~(x, Y,Z, 1 ) - 4~(x, Y,Z,O) where, leaving out the x-argument in the "perturbations", (3.11)

~(x, Y,Z, 3)=//6 +(~ - 1) A6,1t\ \ # + ( 3 - 1) A/~]

[

0 + ~k(Y, Z, G(Y,Z, 6 + (z - 1) A6, 6'+ (~ - 1) A6',/~ + ( 3 - 1)A#, 0" + ( 3 - 1)A 0"))]" 3)=q~(x, Y(x), Z(x), 3), the formula 4~(x, 1)

With the shortened notation r

-~(x,

1

0)= ~ O~/03(x, 3)dz permits us to express the defect d(x, Y, Z) as 0

(3.12)

We only give the expressions of Q2 and Q4 (3.13 a)

Q2(x, Y,Z)= i ku(Y,Z, G(...)) ~6, (...) d3 0

(3.13b)

Q4(x, Y,z)= I k,(l~Z, G(...)) ~077(...) d, 0

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L. Jay

where the missing arguments (...) are identical to those of G in (3.11). A simple differentiation of (3.7e) with respect to 6' and 0" shows that gG

(3.14)

BU

dG

t?6'-t?6'-

(grf~k~)-~gY'

dU

~0"-~30"-

(grfzk")-~'

hence we have 1

QE(X, Y,Z)= --S(ku(grf=ku)-'gr)(Y,Z,G(...))dz

(3.13a')

o -----Q4(X,

Y,Z) gr(e)

1

Q4(x, Y,Z)= - S (ku(gyfz k,)- 1)(Y,Z, G(...))dz.

(3.13 b')

o

The application of [4, Formula (I.14.20)] leads to (3.15)

AZ(x)/

R(x, t, Y(t), Z(t)) d(t, Y(t), Z(t)) dt

+ ~R(x'x~

A(

0

where the resolvent R is given by

R(x, t, y, z)= ~(Y, c~(y, 2), z) tx, t, y, z)

(3.16)

and (Y,,2)(x, t, y, z) denotes the solution at x of (3.9) passing through (y, z) at t. R satisfies R(s, s, y, z)= I and is the solution of the variational equation associated with the system (3.9). In the sequel we will use the abbreviations (3.17)

R(x, t)= R(x, t, Y(t), Z(t)), Q,(t)=Qi(t, Y(t), Z(t)).

d(t)=d(t, Y(t), Z(t)),

b) Let us now consider the first integral in (3.15). By the use of (3.12) we get, after some integrations by parts, (3.18)

f R(x, t) d(t) dt = ~ S~ (x, t) A6(t) 4- S2(x, t) A6'(t)

XO

XO

4- S 3 ( X , t) A#(t)+ S4(x, t) A0"(t) dt ~-

S2(x , t) A 6 ( t ) - ~ t 4 (x, t) A0(t) + S4(x, t) AO'(t) 7=xo+ i a(x, t) d t XO

Collocation methods for index 3 DAE's

415

where we have defined S 1(x, t)--R(x, t )(QII (t))'

(3.19)

S3(X ,

S2(x, t)=R(x, t)(QO(t)),

0 t)=R(x, t)(l+ Q3 (t)), )-~-(

,

S4(x, t)=R(x, t)(QO(t)) , +S3(x,t ) Ap(t) + ~ f - ( x , t)AO(t).

An expression entering in (3.18) is 0S4 X

~ ( , t)= ~t (x, t) (Q~

(3.20)

R(x, t) (Q~(t)),

therefore OR/Ot(x, t) remains to be computed. By using well-known properties of the resolvent, we arrive at (3.21) OR x t

"" /

+~

OR

fy(Y(t), Z(t))

f~(Y(t), Z(t))

\

(x, t, Y(t), Z(t)) (d(t, Y(t), Z(t))," )

where H(t):=G(Y(t), Z(t), 3(0, 3'(0,/~(t), 0"(t)) and we point out that OR/O(y, z) is a bilinear application. Now, we put x :=x o + h in (3.15). The assumption cs= 1 implies that 6(x), 3(x), 0(x), and O(x) vanish at this point. By replacing O'(xo) and O'(Xo) with the help of (3.7d) and by using the relation S2(x,t) = S4(x, t)gy(Y(t)) which is a consequence of (3.13'), we deduce (3.22) xo+h

R (xo + h, t) d(t) d t = S4(x o + h, Xo) ((Agr(q))./~(Xo) + A(grf)(q, ()) XO

OS4

~tt (x~176176176176

xo+ h

~ a(x~

XO

Because of OR/O(y, z)(xo+h, estimated by (3.23)

Xo,y,z)= O(h), the second integral in (3.15) can be

R (Xo + h, Xo) A ( + O (h IIA r/II2 + h IIA (ll 2).

By collecting the previous results and rearranging the terms, we obtain (3.24)

(AY(x~ \AZ (Xo +

+hiLAr

L. Jay

416

where (3.25)

o

(1) = R (x o + h, Xo) [\A(]

\Q4(xo)((Agy(,))./~(Xo) + A(gyf)(r/, ()))

+{ fztq, ~)Qa(xo) \(k~+k.~z)(rl,~,H(xo))Q,*(Xo)-Q4(xo t (2) : -- z(3(y, ~

(3)=

Ag(q)], d

0

(x~+ h, x 0 , r/,

Q4(xo+h) O'(xo+h'

(4)=

~

a(xo+h,t)dt.

Xo

In order to estimate each of these terms, some intermediate results will be required. c) First of all, by expanding the resolvent R(xo + h, Xo) at Xo, we obtain

. - ./

fy(q,O f~(q,O \+O(h z) ky+ku (...) kz+ku (...

with (...)=(r;, ~, H(xo)). Secondly, by differentiating k times the difference of the two collocation polynomials, written in the form (3.2), we arrive at (3.27a)

hkAytk)(xo+th)=l~)(t)Arl+ ~ l~k)(t)AY(xo+cih),

k = 0 .... ,s,

i=1

(3.27b)

hkAZtk)(xo+th)=Itok)(t)A(+~ l~k)(t)AZ(xo+clh),

k = 0 .... ,s,

i=1

(3.27c)

hkAU'k)(xo+th)= ~ I3k'(t)AU(xo+cih),

k=O ..... s - l ,

i=l

and the higher derivatives vanish identically. The proof of [-3, Theorem 6.4] related to R K methods may be adapted in our situation, leading to

hf~ P~A ( + O ( h

(3.28 a)

AY(xo +ci h) =Py Av/+ci

(3.28b)

AZ(xo+c, h)-:P~A(+O (1 'lQyaql'

(3.28c)

AU(xo +c, h)= O 1 IlQyAnTI II+ ~ I1PrAr/tl + ~ tlO~ A(fl + IIP~A(II 9

I[At/I[ + h 2 ][A(II)

+ t'Pr Ar/I, + h ,IAf,,)

Collocation methods for index 3 DAWs By using ~ li(t)= 1 and i i=O

417

li(t)ci=t, these relations inserted into (3.27) yield

i=0

for x in an O(h)-neighbourhood of Xo, (3.29a)

hkaytk)(x) =O (IIQr At/It +h i -,o~ ilprAt/If +h 2 ItQzA~tl +h 2-s'~ IIP~A(II)

(3.29b)

hkAztk'(x)=O( 1 "Qy Ar/ll + IlP~ar/tl + 'IQ=A(II +hX-S~

(3.29c)

hkAUt~'(x)=O(~--gIIQ,Artll +1 IIP~Ar/ll + 1 fIG A~II+ IIP~A~II)

A(II)

where Sjk= 1 if j > k else S~k=O. Since A6(x)=AY'(x)--Af(Y(x), Z(x)), A#(x) = AZ' (x)-- A k (Y(x), Z (x), U (x)), and A 0(x) = - A g (Y(x)), we have (3.30)

A~(~)--O(h lIQ,A.ll + liP,A.ll + iIQ=Ar + ll~ A:il)

A.(x)= o (~ i,~. A.,, +~ ,,~ A.i, +88 ,,~=a:i, + i,ez A~,,) A0..(x): o (~ ,,~. A~,, +~ ,t~ A~,, +,l~z A:,, + ,i~ A:l,) Thirdly, we define (~/, ~', ~ as consistent values close to (r/, if, v) uniquely determined by the equations (2.2), Py(r/-0) = 0 and P~(~-~')=0. The hypothesis (3.1a, b) shows that Qy(rl-71)=O(h r ) and Qz(~-~)=O(h mm(r " ' x)). As in (3.7), we denote by a-(x), Ft(x), and ~7(x) the defects of the collocation polynomials passing through (if, ~', v'). These defects vanish at the collocation points Xo+Ci h, therefore we have a'(x)= O(hS), a-'(x) = O(h ~- i), ~(x)= O(h~), and g(x)= O(h ~+1), ~7'(x)= O(hS), ~"(x)=O(h ~-~) by also taking into account the consistency of O, i.e., 0Ix0)= - g ( 0 ) = 0 . These estimates and the application of (3.30) to r / - 0 and ~ - ~ lead to (3.31)

(X) = (~ (X) - - 3(X)) + a-(X) = O (h min (* - 1 . . . . )), ~ ' ( X ) = O ( h mint*- 2,~r - 1 , s - 1)),

]A(X)_~_O(hmin(r-2,~-l,s)),

O,,(x)=O(hmin(r-2 ....

-1)).

d) With these preparations we are now able to treat each term of (3.25). From (3.30) and (3.31) we deduce that (3.32)

O_4(xo)g,f=Qza~= -o=a~ + O (lIQr Ar/N + h liPr Ar/I I + h min~*- 3,~- 2,s- 2)+, iIQz Ar

+ I N G A~l12+h2 HP~A~,I)

L. Jay

418 and (3.33a)

Q~ (Xo + h) Q4 (Xo + h) = - ku(gyf~ k~)- l(t/, ~, G (/I, ~)) -1-O (h)

(3.33b)

P~(xo+ h) Q4(xo + h) = O(h rnin(r- 2. r -

1, s - 1))

+O(hl_fllQyA~)l + l l l g A t / i ] +~tlQzA(II 1 + IIP~A(II). By collecting the results (3.26) and (3.32), we get (3.34) (1) =(PY At/+P~A( h fz P~A q]

+[O(h tIA t/I] + hmin(r- a'K-2's- 2)+ 2t1 Qz A(tl +

ItQz

A~tl 2+ hE tlPzA (I[)\

[~0 (]tAt/t] + h min(~-3,x- 2's- 2)+ 1 IIQz A~[I +~-t[Qz A~[I2 + h ItPzA ~[I)/" | Because of d(xo, t/, 0 = O(h) we have the rough estimate (2)= O (h ]]At/]t). By setting a:= ~ / ~ ) ( 1 ) = i=1 (3.35)

-

-

ltl)'tx o ~J, a consequence of

a AY'(xo+h)= -~QyAt/+f~P~A(+O(][At/[f+hJlA(t[)

and A0'(x) = - A [gy(Y(x)) Y'(x)] is (3.36)

A0'(Xo + h)=~-a gy Qy At/+ O([IA t/[I + h [IA(I[).

Combined with (3.33) we obtain the following decomposition for Q4(xo + h) A O'(x o + h) entering in (3)

a (3.37 a) Q~(xo + h) Q4(xo +h) A 0'(Xo + h)= - ~ SQy At/+ O(t[At/+ h IIA(II) (3.37b) P~(Xo + h) Q4 (Xo + h) A 0' (x o + h) = O(llOy At/It + h fIPr At/{I-{--hmin('r-3'K- 2,s- 2)+ 2 [tQ, A~][ + t]Q~ A ( H 2 + h2 I[PzA(tl). As in the proof of [5, Theorem VI.7.9], a(xo+h, t) in (4) can be integrated s 1, yielding by the use of the quadrature formula {bi, c i}i= (3.38)

(4) ----- ~ i= !

bi a(x 0 +

h, Xo + cl h) + err(a). ,e =0

Collocation methods for index 3 DAE's

419

The quadrature error is estimated by

e r r ( a ) = O ( hp+l"

(3.39)

maxt~txo,xo+hl~ ' P a ( x ~

)

From (3.29) it follows that (3.40) (4)= O(h p-~ ][Qy At/II + h p-s+1 liPyA~/lt + h p-s+ 1 IlQzA~[t q-h r-s+2 [IPzA([I). Insertion of the expressions (1), (2), (3.37a, b), and (4) into (3.24) gives the desired result, except that if p = s, the term O (IIQy A q tl + h IIQ~ A ~ II) in A Y(Xo + h) coming from (4) has to be neglected in view of (3.28a) for i=s, and the term PyAq entering in P~(Xo+ h)AZ(x o + h)is O(h ItPr A q fl), because of (3.41) P~(xo+c,h) AZ(xo + c , h ) = P ~ A ( +O(lIQyArlll +hllPyAqll +hllA(ll) which can be proven in the same way as (3.28).

[]

4. The local error

We consider one step of a collocation method (2.4) with consistent initial values (Yo, Zo, Uo). The local error (4.1) 6 yh(xo)= yx -- y(xo + h),

6 zn(xo)= Zl -- Z(Xo + h ), 6uh(xo)=Ul -- U(Xo + h)

can be estimated as follows: Theorem 4.1. Let us assume that s > 2 and that H 1 holds. Then we get (4.2)

(~yh(xo)=O(hS+l),

Py(xo+h) t~yh(xo)=O(hmin(p's+l)+l),

6zh(Xo)=O(h~), 6 Uh(Xo) = 0 (h~- 1)

P~(xo+h)6Zh(Xo)=O(h~+l),

where p is the order of the underlying quadrature formula. Pr(x), Pz(x) are the projectors (3.6) evaluated at the exact solution of (2.1) at x. I f in addition H 2 is satisfied, we have (4.3)

r

Pz(xo+h) 6zh(Xo)~-O(hmin(p'2s-2)+l).

Remark. If the function k of (2.1) is linear in u, then instead of (4.3) we get (4.3')

Pz(xo + h) 6 Zh(Xo) = O(h p + 1).

Proof. The interpretation of the collocation solution in terms of one corresponding I R K method (see Theorem 3.1 for more details) allows us to apply I-3, Lemma 6.3] which leads to (4.2). The results (4.3) can be found quite easily with the same techniques used in the proof of Theorem 3.2. Instead of computing the difference between two collocation polynomials with distinct initial values, we must estimate the differ-

420

L. Jay

ence between a collocation polynomial and the exact solution of (2.1), here with identical initial values. The exact solution has no defect (see (3.7)). The defect of the collocation polynomials vanishes at the collocation points Xo + c~ h, therefore for x~ [Xo, Xo + hi we have 6(x) = O(hS), 6'(x) = O(h ~- 1), /x(x)= O(h~), and O(x)=O(h~+l), O'(x)=O(h~), O"(x)=O(h ~-~) by consistency of the initial value Yo, i.e., 0 ( X o ) = - g ( y 0 ) = 0 . It can be easily shown that the derivatives of the collocation polynomials are uniformly bounded (see [5, Theorem VI.7.8] for an equivalent result concerning index 2 systems). With similar formulas to (3.15) and (3.22) we finally arrive at (4.4) (y(xo + h ) - Y(xo + h)] \z (x o + h)-- Z (x o + h)] =

xo+h

S cr(x o + h, t) d t - S 4 (Xo + h, Xo + h) O'(x o + h)

XO

=O(hP+')+[ 1 0 0) \ o~ k . ( g r f ~ k . ) - ' ( y , z, G(y, z, r6, r&', r/z, tO")) dr. where y, z, 6, 6', #, 0", and O' are evaluated at x o + h in the last expression.

[]

5. Proof of Theorem 2.2 We only outline the main points. Following the proof given in [3, Theorem 6.4], we denote two neighbouring collocation solutions by {jT.,~.}, {j~,,~,} and their difference by A y . = y . --j?,, A z , = ~ . - ~ , . In a first step it can be shown that (see [3, Theorem 6.4] or apply the second step with m = n = 0) (5.1)

llAy, l l < C l h s+l ,

IIAz.ll